Ultra-strong interaction of metamaterial plasmons with photons in a terahertz photonic crystal cavity

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Cavity (quantum) electrodynamics investigates the strong interaction of matter with photons in a resonant cavity. It has long been studied intensively to investigate fundamental quantum properties in physics such as Schrödinger's cat states and entanglement of photons, and to explore approaches for quantum information processing. Initially, strong coupling was only investigated with natural atoms, while in recent years, it has been realized experimentally in a variety of material systems, employing interband and inter-subband transitions in semiconductor quantum wells and quantum dots, spin resonances in magnetic materials, and bosonic excitations such as cyclotron transitions in 2D electron gases and molecular vibrational transitions in polymers. We have recently demonstrated strong light-matter coupling of metasurfaces, a class of artificial materials, with terahertz photons in a one-dimensional photonic crystal cavity. We have employed plasmonic metallic structures which have been studied extensively in the literature, not least because of their potential for the realization of chemical and biological sensors, optical filters and modulators. With their huge dipole moments, the unit cells of the metasurfaces interact efficiently with the photons in the cavity, forming plasmon-photon polaritons with an enormous Rabi splitting of the upper and lower polariton branches. In spite of this being a classical electrodynamic system, it has turned out that the terminology employed in cavity quantum electrodynamics is useful and applicable.Based on our findings, we intend to pursue two lines of research. One is devoted to a better understanding and an enhancement of this form of light-matter interaction. We want to answer open questions regarding the unexpectedly strong coupling of Babinet-complementary metasurfaces with the photons, and then study the coupling of ‘dark atoms’ with the cavity photons via inter-unit-cell interaction in the metamaterial. In order to achieve even stronger coupling – reaching high into the ultrastrong-coupling regime – we then focus on the development of terahertz Fabry-Perot cavities which exhibit a smaller fundamental-mode volume than the photonic crystal cavities studied by us so far. With this reduction of the mode volume, we aim to demonstrate an even stronger Rabi splitting of the polariton branches than obtained until now. In the second line of work, we develop capabilities to actively switch the interaction strength. The literature has shown ways to modify the optical properties of metasurfaces by external control parameters. This should work even better with the strong interaction in a cavity. We will focus on metasurfaces built from split-ring-resonators, and adopt concepts to change their properties by a bias voltage applied to the unit cells, or by the absorption of laser radiation impinging on them. With this research, we intend to prepare switching platforms for future use in applications.